Generally, a punch is a fist thrown at someone with the target area being say, from the waist up. Examples: A1 tries to punch B2 and hits him in the face or short C4 tries to punch tall F5 and hits him in the chest. These are considered flagrant and the offender is ejected.

Situation: A1 is displaced by B3 and both fall to the floor. Official calls the foul. B3 rises and A1, in a sitting position, clearly retaliates by swinging a straight-armed fist from the side, hitting the knee of B3. B3 stumbles and falls to the floor in great pain.

To truly give a ruling, you had to be there but... given the description, would you go with intentional or flagrant foul?

Is the landing spot of the fist relevant?

__________________
If some rules are never enforced, then why do they exist?